Small integrated circuit (IC) elements, dies, or chips, such as a radio frequency (RF) amplifier on a radio chip, are sometimes connected to an off-chip device. These off-chip devices include other semiconductor or hybrid circuits as well as other integrated circuits such as an off-chip filter. One way of attaching the IC to an off-chip device utilizes a wire bonding technique. This entails using a wire bonding machine to fuse small wires to the contact points or bond pads of small IC chips at one end and attach to an off-chip device at the opposite end.
In both hybrid circuits and IC chips, one or more devices may be carried by a ceramic substrate. For example, high frequency signals, such as RF signals, can be transmitted via a transmission line structure formed on one of the devices. The RF signals may be transmitted to any number of off-chip devices such as a surface acoustical wave (SAW) filters or the like. The transmission line is usually formed by any two conductors or wires which are insulated from each other.
It is at the interface between the transmission line of a off-chip device and a small IC die of the RF device that a bond wire connection is made. The transmission line of the off-chip device is fully capable of handling radio frequencies, yet a crosstalk problem is often encountered when attempting to connect an IC die to the off-chip transmission line.
When any two or more circuits are interconnected on different electrical devices having substrates, such as printed circuit boards, IC's or other types of carriers, by a standard transmission line, a certain amount of signal from one circuit may couple into an otherwise isolated circuit. The resulting induced signal is called crosstalk. This situation can be a problem, for example, in a radio transceiver, when an RF high frequency signal source, such as an antenna received radio frequency RF signal, is routed from an RF amplifier in one substrate, to an RF filter, in another substrate. Under these conditions crosstalk can occur.
If one of the two conductors of the transmission line coupling the circuits is used as a grounding conductor, the circuit is referenced as single-ended and the transmission line is a single-ended transmission line. Otherwise, two non-grounded conductors form a differential transmission line for a differential circuit. The geometry of the two conductors, and their relative positions, are set to establish a characteristic impedance to properly match the impedance from the first differential circuit to the subsequent differential circuit. A differential signal is applied across the two conductors by one differential generating circuit, and the signal travels down the transmission line to the differential receiving circuit, where the signal is measured as the difference between the two conductors. In other words, a differential circuit generates or receives a pair of complementary signals in a phase-inverted relation with each other, known together as a single differential signal.
The substitution of a differential or wire bond pair transmission line for a single-ended wire bond transmission line can greatly reduce at least one type of crosstalk or coupling called common-mode impedance coupling. Common-mode impedance coupling in a single-ended (non-differential) transmission line is caused by a non-zero parasitic impedance, generally called a ground-return impedance, comprised of reactive and resistance components, unintentionally shared by two or more otherwise isolated circuits. This common impedance causes crosstalk. Therefore, by reducing; the common or shared impedance of the ground conductor with a non-grounded conductor, common-mode impedance coupling is reduced. In addition to common impedance coupling between circuits, there can also be capacitive (electric) and inductive (magnetic) coupling between the two conductors of the same transmission line if the two conductors are close together.
In the case of two adjacent; transmission lines, differential transmission lines can also reduce the capacitive, or electric field, coupling and inductive, or magnetic field, coupling between the differential transmission lines, relative to single-ended transmission lines, if the distance between the, two transmission lines is much larger than the separation between the conductors of one of the transmission lines. However, space is generally limited in these miniaturized applications, such as in a radio. Therefore, these differential transmission lines must be close together. This close situation increases the magnetic coupling and decreases the advantage of differential lines over single-ended lines.
FIG. 1 illustrates a prior art figure depicting two adjacent circuits 101, 102, each of which is implemented on both substrates 103, 104. The wire bond pairs 110, 111, as implemented in the prior art, are substantially parallel. The RF signal source is represented by the voltage Vi 120. The crosstalk is represented by the voltage Vx 121 on the adjacent circuit 102. FIG. 2 shows a typical plot of crosstalk versus frequency as a ratio of the voltages Vx to Vt. FIG. 2 is representative of the crosstalk present between two such otherwise isolated circuits 101, 102 as shown in FIG. 1, with a separation of approximately 200 microns.
A further reduction in crosstalk, over the plain differential line, for the two close conductors of the same differential line or for two close differential lines, can be achieved by twisting the two conductors to form a twisted-pair differential transmission line. This type of line is commonly implemented with two insulated round wires which are twisted about each other. This type of line can reduce crosstalk by reducing the inductive, or magnetic field, coupling. The crosstalk reduction is achieved by reducing the magnetic loop area of the line, and by changing the orientation of the magnetic field continuously over the length of the line. A problem associated with this type of twisted pair is that insulation is required since the wires are in intimate contact. This prevents a short which would defeat the benefits and crosstalk reduction of the ability of the twisted pair.
An un-twisted pair defines a magnetic, or current, loop area over the entire length of the differential transmission line, with the wires being the long sides of a "rectangle". The area of such a loop defines the amount of current that can be induced in the wires by an external magnetic field, such as from an adjacent conductor.
By twisting or crossing the wires, the loop area is minimized. The dimensions of the wire, and the number of twists per inch define the transmission lines characteristic impedance. Furthermore, the remaining loop is now twisted down the length of the transmission line, so that the normal to the magnetic loop area traces a helix or spiral.
When interconnecting miniature circuits, such as printed circuits on a printed circuit board or a flexible circuit substrate, ICs on a semiconductor substrate, or hybrid circuits, the concept of twisted-pair differential transmission lines can be applied to reduce crosstalk. However, when connecting miniature circuits on two separate substrates with typical wire bonds, all the interconnections between substrates, even if the circuits are twisted-pair differential, are implemented by bond wires that are substantially parallel, as shown in FIG. 1, and these parallel interconnections can lead to significant crosstalk. As seen in prior art FIG. 2, a graph of the representative crosstalk function as shown in FIG. 1 is illustrated versus frequency. As can be dearly seen, the crosstalk function increases and then remains relatively constant with increasing frequency.
Therefore, it should be evident that there is a need to eliminate crosstalk easily and at a low cost without adding an excessive number of components. In order to meet this need, the device should be comprised of common materials and manufactured with tools and machinery capable of quickly and inexpensively creating a device to eliminate circuit coupling and cross-talk. This provides better circuit operation and an increase in overall efficiency between electrical circuits in close proximity.